The defendant should've claimed self defence, even if he really didn't kill his father.
He was acquitted, but considering the evidence, his chances of being acquitted when claiming he didn't kill his father were extremely low.
He would've had a much better chance if he claimed self defence. The father was known to be physically abusive. If he said he was defending himself, everyone would've believed him, and he would've had a much better chance of being acquitted.
He still may have been convicted. The jury may have decided that although it was self defence, killing him was excessive. However, if he was convicted, it would've been manslaughter, which would've only resulted in a relatively short prison sentence.